The present invention relates to a spool for a retractor for a seat belt arrangement and particularly to a spool for a load limiting seat belt retractor having a torsion bar.
A seat belt safety restraint has seat belt webbing wound on a spool, which is mounted for rotation about the spool""s longitudinal axis to wind in or pay out belt webbing. The spool is biased in a winding-up direction by a spiral spring. When a sensor detects a crash, for example by detecting acceleration or deceleration above a predetermined threshold, a locking mechanism is activated to lock the spool against rotation and prevent further pay-out of belt webbing, thus restraining a vehicle occupant against forward motion.
The use of a torsion bar is well known for load limiting seat belt retractor and is arranged in the force path to absorb some of the crash forces to reduce the effects of the seat belt restraint on the vehicle occupant in a crash. A torsion bar is malleable and deforms plastically as it twists without shearing and allows a small amount of pay-out of seat belt webbing after the spool has been locked against rotation. A torsion bar is usually arranged coaxial with the spool having one end connected to the spool and the opposite end arranged to be locked against rotation in the event of a crash, e.g. by a locking pawl engaging a toothed ratchet wheel attached to the torsion bar.
Typically, to accommodate a torsion bar, a seat belt retractor spool is made hollow and may be assembled from two parts placed around the torsion bar and riveted or staked together. This requires dedicated equipment on the retractor production line and the process of fixing the two spool parts together is time consuming. The resultant seat belt retractor spool is not as strong as would be desired since there is no substantial support for the torsion bar around its main diameter. When a load is applied in a crash, the webbing pin used to retain seat belt webbing during the crash can bend and is forced into a gap between the torsion bar gap between the torsion bar and the spool member. Since both ends of the torsion bar are supported, the torsion bar bends at its middle, the weakest point. This bending changes the direction of the load on the webbing pin/spool member and the spool is weakened and may fail prematurely. This is especially so since a webbing pin, traditionally sewn onto the end of the belt webbing to hold the end in the spool, often is able to push against the torsion bar under load conditions.
According to the present invention, there is provided a seat belt retractor spool assembly comprising a cylindrical central spool member having a longitudinal axis and a hollow section along said axis and having a belt webbing entrance slot and a belt webbing exit slot, a torsion bar is located in the hollow section of the spool member, a gear wheel for rotatably driving the spool and a filler plate is shaped to fit into the entrance slot and prevents relative movement of the torsion bar, the gear wheel and the cylindrical central spool member.
A seat belt retractor spool according to the invention can be made more cheaply and more quickly since there are fewer parts to assemble and less manufacturing steps, particularly compared to the riveted spools of the prior art, and the spool assembly is stronger under load.